The term OPS-lasers, as used herein, refers to a class of vertical-cavity surface-emitting semiconductor lasers wherein optical gain is provided by recombination of electrical carriers in very thin layers, for example, about 150 .ANG.ngstrom units (.ANG.) or less, of a semiconductor material. These layers are generally termed quantum-well (QW) layers or active layers.
In an OPS-laser, several QW layers, for example, about fifteen, are spaced apart by separator layers also of a semiconductor material, but having a higher conduction band energy that the QW layers. This combination of active layers and separator layers may be defined as the gain-structure of the OPS-laser. The layers of the gain-structure are epitaxially grown. On the gain-structure is an epitaxially-grown multilayer mirror-structure, often referred to as a Bragg mirror. The combination of mirror-structure and gain-structure is referred to hereinafter as an OPS-structure.
In an (external cavity) OPS-laser, another (conventional) mirror, serving as an output-coupling mirror is spaced-apart from the OPS-structure, thereby forming a resonant cavity with the mirror-structure of the OPS-structure. The resonant cavity, accordingly, includes the gain-structure of the OPS-structure. The mirror-structure and gain-structure are arranged such that QW layers of the gain-structure are spaced apart by one half-wavelength of the fundamental laser wavelength, and correspond in position with antinodes of a standing-wave of the fundamental laser-radiation in the resonator. The fundamental-wavelength is characteristic of the composition of the QW layers.
Optical pump-radiation (pump-light) is directed into the gain-structure of the OPS-structure and is absorbed by the separator layers of the gain-structure, thereby generating electrical-carriers. The electrical-carriers are trapped in the QW layers of the gain-structure and recombine. Recombination of the electrical-carriers in the QW layers yields electromagnetic radiation of the fundamental-wavelength. This radiation circulates in the resonator and is amplified by the gain-structure thereby generating laser-radiation.
OPS-lasers have often been used in the prior art as a means of conveniently testing QW structures for later use in electrically-pumped semiconductor lasers. More recently, OPS-lasers have been investigated as laser-radiation sources in their own right. The emphasis of such investigation, however, appears to be on providing a compact, even monolithic, device in keeping with the generally compact nature of semiconductor lasers and packaged arrays thereof.
The gain-structure of OPS-structures may be formed from the same wide range of semiconductor-materials/substrate combinations contemplated for diode-lasers. These include, but are not limited to, InGaAsP/InP InGaAs/GaAs, AlGaAs/GaAs, InGaAsP/GaAs and InGaN/Al.sub.2 O.sub.3, which provide relatively-broad spectra of fundamental-wavelengths in ranges, respectively, of about 960 to 1800 nanometers (nm); 850 to 1100 nm; 700 to 850 nm; 620 to 700 nm; and 425 to 550 nm. There is, of course, some overlap in the ranges. Frequency-multiplication of these fundamental-wavelengths, to the extent that it is practical, could thus provide relatively-broad spectra of radiation ranging from the yellow-green portion of the electromagnetic spectrum well into the ultraviolet portion.
In conventional solid-state lasers, fundamental-wavelengths, and, accordingly, harmonics thereof (produced by frequency-doubling or frequency-mixing) are limited to certain fixed wavelengths characteristic of a particular dopant in a particular crystalline or glassy host, for example, the well-known 1064 nm wavelength of neodymium-doped yttrium aluminum garnet (Nd:YAG). While one of these characteristic wavelengths may be adequate for a particular application, it may not be the optimum wavelength for that application.
OPS-lasers provide a means of generating wavelengths, in a true CW mode of operation, which can closely match the optimum wavelength for many laser applications, in fields such as medicine, optical metrology, optical lithography, and precision laser machining. Prior-art OPS-lasers, however, fall far short of providing adequate power for such applications. It is believed that the highest fundamental output-power that has been reported, to date, for an OPS-laser is 700 mW at a wavelength of about 1000 nm (Kuznetsov, et al., IEEE Photonics Tech. Lett 9, 1063 (1997)). For an intracavity frequency-doubled OPS-laser, it is believed that highest output-power that has been reported is 6 mW at a wavelength of about 488 nm (Alford et al. Technical Digest of the IEEE/OSA Conference on Advanced Solid State Lasers, Boston Mass., Feb. 1-3, 1999, pp 182-184). It believed that there has been no report to date of generation of continuous wave (CW) ultraviolet (UV) radiation in an OPS-laser, either directly or by frequency-multiplication.
However flexible an OPS-laser may be in potentially offering a wide selection of wavelengths, in order to be competitive in applications in which solid-state and other lasers are currently used, at least an order-of-magnitude, and preferably two orders-of-magnitude increase in power over that offered by prior-art OPS-lasers is required. This power increase must also be achieved without sacrifice of output-power stability and beam-quality. Further, in order to be applicable in the broadest range of applications the range of OPS-laser wavelengths available at high-power and with high beam-quality must be extended into the UV region of the electromagnetic spectrum, preferably to wavelengths less than 300 nm.